The conversion of synthesis gas into hydrocarbons by the Fischer-Tropsch process has been known for many years but the process has only achieved commercial significance in countries where unique economic factors prevail. The growing importance of alternative energy sources such as coal and natural gas has focussed renewed interest in the Fischer-Tropsch process as one of the more attractive direct and environmentally acceptable routes to high quality transportation fuels.
Many metals, for example cobalt, nickel, iron, molybdenum, tungsten, thorium, ruthenium, rhenium and platinum are known to be catalytically active, either alone or in combination, in the conversion of synthesis gas into hydrocarbons and oxygenated derivatives thereof. Of the aforesaid metals, cobalt, nickel and iron have been studied most extensively. Generally, the metals are used in combination with a support material, of which the most common are alumina, silica and carbon.
The use of cobalt as a catalytically active metal in combination with a support has been described in, for example, EP-A-127220, EP-A-142887, GB-A-2146350, GB-A-2130113 and GB-A-2125062. EP-A-127220, for example discloses the use of a catalyst comprising (i) 3-60 pbw cobalt, (ii) 0.1-100 pbw zirconium, titanium, ruthenium or chromium, per 100 pbw silica, alumina or silica-alumina, (iii) the catalyst having been prepared by kneading and/or impregnation.
Our earlier patent EP 261870 describes a composition for use after reductive activation as a catalyst for the conversion of synthesis gas to hydrocarbons comprising as essential components (i) cobalt either as the elemental metal, oxide or a compound thermally decomposable to the elemental metal or oxide and (ii) zinc in the form of the oxide or a compound thermally decomposable to the oxide. The resultant catalysts, in contrast to many prior art cobalt-containing catalysts, are more selective to hydrocarbons in the C5-C120 range and can be very selective to a waxy hydrocarbon product. These catalysts may also contain in elemental form or oxide form one or more of the following metals as promoters: chromium, nickel, iron, molybdenum, tungsten, zirconium, gallium, thorium, lanthanum, cerium, ruthenium, rhenium, palladium or platinum suitably in amount up to 15% w/w. Exemplified compositions included chromium, zirconium, gallium and ruthenium as promoters.
U.S. Pat. No. 4,826,800 describes a process for preparing a catalyst comprising cobalt and zinc oxide for use after reductive activation as a catalyst in the conversion of synthesis gas to hydrocarbons. The catalyst is prepared by mixing a solution, of a soluble zinc salt and a soluble cobalt salt with a precipitant such as ammonium hydroxide or ammonium carbonate and recovering the precipitate. The ratio of carbonate to metal is high in the described method, which has been found detrimental to the strength of the catalyst.
U.S. Pat. No. 5,345,005 relates to a Cu—Zn catalyst on alumina for the preparation of alcohols by hydrogenation of e.g. a ketone. In a comparative example, the preparation of a Cu—Zn—Co catalyst on alumina is described, wherein use is made of soda ash. However, the use of soda ash is found to be potentially detrimental to the strength of the catalyst. The particle size distribution range within which 90% of the volume of the Cu—Zn—Co catalyst described in U.S. Pat. No. 5,345,005 lies, is not specified. It is however expected that the use of soda ash in the preparation of the catalyst leads to a broadening in the particle size distribution.
U.S. Pat. No. 5,945,458 and U.S. Pat. No. 5,811,365 describe a Fischer-Tropsch process in the presence of a catalyst composition of a group VIII metal, eg. cobalt, on a zinc oxide support. Such a catalyst is made by first preparing the support by adding a solution of zinc salt and other constituents to an alkaline bicarbonate solution. Next, the precipitate is separated from the bicarbonate solution by filtration to form a filter cake, which can thereafter be dried, calcined and loaded with the group VIII metal. The catalyst material is then formed into tablets, which tablets are crushed to form particles with a size of 250-500 μm, that can be used in a Fischer-Tropsch process. Additional post-treatments such as crushing, are required in order to obtain a catalyst powder with good strength properties. However, the obtained average particle size; as indicated above, is still relatively large. Moreover, crushing results in a broad particle size distribution and catalysts with such a large particle size and a broad particle size distribution tend to be less suitable for processes involving a bubble column, a slurry phase reactor or a loop reactor.
WO-A-01/38269 describes a three-phase system for carrying out a Fischer-Tropsch process wherein a catalyst suspension in a liquid medium is mixed with, gaseous reactants in a high shear mixing zone, after which the mixture is discharged in a post mixing zone. Thus mass transfer is said to be enhanced. As suitable catalysts inter alia cobalt catalysts on an inorganic support, such as zinc oxide are mentioned. The surface area of the support used for the preparation of these known catalysts is less than 100 g/m2. These prior art cobalt based catalysts can be prepared by depositing cobalt on a suitable support, such as a zinc oxide support, by impregnation methodology. Other conventional preparation methods include precipitation routes, which typically involve crushing of a hard filter cake of catalyst material, resulting from the catalyst preparation process, into small particles.
More recently WO 03/090925 describes a Fischer-Tropsch catalyst comprising particles of a cobalt and zinc co-precipitate having specific volume average particle size and particle size distributions. The catalysts essentially consist of cobalt and zinc oxide but may also contain other components commonly employed in Fischer-Tropsch catalysts such as ruthenium, hafnium, platinum, zirconium, palladium, rhenium, cerium, lanthanum, or a combination thereof. When present such promoters are typically used in a cobalt to promoter atomic ratio of up to 10:1.
EP 221598 describes supported catalysts comprising a metal component of iron, nickel or cobalt promoted by zirconium and in addition a noble metal from Group VIII of the Periodic Table. The catalysts are suitable for the preparation of hydrocarbons from carbon monoxide and hydrogen. Preferred noble metals include platinum or palladium and the catalysts are most suitably supported on silica or alumina.